Secretion of methanol-insoluble heat-stable enterotoxin (STB): energy- and secA-dependent conversion of pre-STB to an intermediate indistinguishable from the extracellular toxin.

The methanol-insoluble, heat-stable enterotoxin of Escherichia coli synthesized by clinical strains or strains that harbor the cloned gene was shown to be an extracellular polypeptide. The toxin (STB) was first detected as an 8,100-Mr precursor (pre-STB) that was converted to a transiently cell-associated 5,200-Mr form. Proteolytic conversion of pre-STB to STB was shown to be inhibited by the proton motive force uncoupler carbonyl cyanide m-chlorophenylhydrazone and did not occur in a secA background. After STB was detected as a cell-associated molecule, an extracellular form with identical electrophoretic mobility became apparent. The results suggest that there is no proteolytic processing during the mobilization of STB from the periplasm to the culture supernatant. The determined amino acid sequence of STB coincides fully with the 48 carboxy-terminal amino acids inferred from the DNA sequence. The 23 amino-terminal residues inferred from the DNA sequence were absent in the mature toxin.